decompression sickness

Decompression Sickness

Definition

Decompression sickness (DCS) is a dangerous and occasionally lethal condition caused by nitrogen bubbles that form in the blood and other tissues of scuba divers who surface too quickly.

Description

According to the Divers Alert Network (DAN), a worldwide organization devoted to safe-diving research and promotion, less than 1% of divers fall victim to DCS or the rarer bubble problem called gas embolism, air embolism, or arterial gas embolism (AGE). A study of the United States military community in Okinawa, where tens of thousands of sport and military dives are made each year, identified 84 DCS and 10 AGE cases in 1989–95, including nine deaths. This translated into estimates of one case in every 7,400 dives and one death in every 76,900 dives. DCS symptoms can be quite mild, however, and many cases certainly go unnoticed by divers.

At times the terminology adopted by writers on DCS can be confusing. Some substitute the term decompression illness (DCI) for DCS. Others treat DCI as a label encompassing both DCS and AGE. An older term for DCS is caisson disease, coined in the nineteenth century when it was discovered that bridge construction crews working at the bottom of lakes and rivers in large pressurized enclosures (caissons) were experiencing joint pain (a typical DCS symptom) on returning to the surface.

Causes and symptoms

The air we breathe is mostly a mixture of two gases, nitrogen (78%) and oxygen (21%). Unlike oxygen, nitrogen is a biologically inert gas, meaning that it is not metabolized (converted into other substances) by the body. For this reason, most of the nitrogen we inhale is expelled when we exhale, but some is dissolved into the blood and other tissues. During a dive, however, the lungs take in more nitrogen than usual. This happens because the surrounding water pressure is greater than the air pressure at sea level (twice as great at 33 ft [10 m], for instance). As the water pressure increases, so does the pressure of the nitrogen in the compressed air inhaled by the diver. Because increased pressure causes an increase in gas density, the diver takes in more nitrogen with each breath than he or she would at sea level. Instead of being exhaled, however, the extra nitrogen safely dissolves into the tissues, where it remains until the diver begins his or her return to the surface (under some circumstances the extra nitrogen can cause nitrogen narcosis, but that condition is distinct from DCS). On the way up, decompression occurs (in other words, the water pressure drops), and with the change in pressure, the extra nitrogen gradually diffuses out of the tissues and is delivered by the bloodstream to the lungs, which expel it from the body. If the diver surfaces too quickly, however, potentially dangerous nitrogen bubbles can form in the tissues and cause DCS. These bubbles can compress nerves, obstruct arteries, veins, and lymphatic vessels, and trigger harmful chemical reactions in the blood. The precise reasons for bubble formation remain unclear.

How much extra nitrogen enters the tissues varies with the dive's depth and duration. Dive tables prepared by the U.S. Navy and other organizations specify how long most divers can safely remain at a particular depth. If the dive table limits are exceeded, the diver must pause on the way up to allow the nitrogen to diffuse into the bloodstream without forming bubbles; these pauses are called decompression stops, and are carefully calibrated. DCS can occur, however, even when a diver obeys safe diving rules. In such cases, the predisposing factors include fatigue, obesity, dehydration, hypothermia, and recent alcohol use. People who fly or travel to high-altitude locations without letting 12-24 hours pass after their last dive are at risk for DCS as well because their bodies undergo further decompression. This is true even when flying in commercial aircraft. Many travelers are unaware that to save money on fuel the cabin pressure in commercial aircraft is set much lower than the pressure at sea level. At 30,000 ft (9,144 m), for instance, cabin pressure is usually equivalent to the pressure at 7,000-8,000 ft (2,133-2,438 m) above sea level, a safe setting for everyone but recent divers. Exactly how long a diver should wait before flying or traveling to a high-altitude location depends on how much diving he or she has done and other considerations. If there is uncertainty about the appropriate waiting period, the sensible course of action is to let the full 24 hours pass.

Because the nitrogen bubbles that cause DCS can affect any of the body's tissues, including the blood, bones, nerves, and muscles, many kinds of symptoms are possible. Symptoms can appear minutes after a diver surfaces, and in about 80% of cases do so within eight hours. Pain is often the only symptom; this is sometimes called the bends, although many people incorrectly use that term as a synonym for DCS itself. The pain, which ranges from mild to severe, is usually limited to the joints, but can be felt anywhere. Severe itching (pruritis), skin rashes, and skin mottling (cutis marmorata) are other possible symptoms. All of these are sometimes classified as manifestations of type 1 or "mild" DCS. Type 2 or "serious" DCS can lead, among other things, to paralysis, brain damage, heart attacks, and death. Many DCS victims, however, experience both type 1 and type 2 symptoms.

Diagnosis

Diagnosis requires taking a medical history (questioning the patient about his or her health and recent activities) and conducting a physical examination.

Treatment

DCS is treated by giving the patient oxygen and placing him or her in a hyperbaric chamber, an enclosure in which the air pressure is first gradually increased and then gradually decreased. This shrinks the bubbles and allows the nitrogen to safely diffuse out of the tissues. Hyperbaric chamber facilities exist throughout the United States. No matter how mild one's symptoms may appear, immediate transportation to a facility is essential. Treatment is necessary even if the symptoms clear up before the facility is reached, because bubbles may still be in the bloodstream and pose a threat. DAN maintains a list of facilities and a 24-hour hotline that can provide advice on handling DCS and other diving emergencies.

Prognosis

DCS sufferers who undergo chamber treatment within a few hours of symptom onset usually enjoy a full recovery. If treatment is delayed the consequences are less predictable, although many people have been helped even after several days have passed. A 1992 DAN report on diving accidents indicated that full recovery following chamber treatment was immediate for about 50% of divers. Some people, however, suffer numbness, tingling, or other symptoms that last weeks, months, or even a lifetime. In the Okinawa study, six of the 94 patients experienced "long-lasting" symptoms even after repeated chamber treatments.

Key terms

Gas embolism — The presence of a gas bubble in the bloodstream that obstructs circulation.

Hyperbaric chamber — A sealed compartment in which air pressure is gradually increased and then gradually decreased, allowing nitrogen bubbles to shrink and the nitrogen to safely diffuse out of body tissue.

Lymphatic vessels — Vessels that carry a fluid called lymph from the tissues to the bloodstream.

Nitrogen narcosis — Also called "rapture of the deep," the condition is caused by increased nitrogen pressure at depth and is characterized by symptoms similar to alcohol intoxication.

Prevention

The obvious way to minimize the risk of falling victim to DCS is to follow the rules on safe diving and air travel after a dive. People who are obese, suffer from lung or heart problems, or are otherwise in poor health should not dive. And because the effect of nitrogen diffusion on the fetus remains unknown, diving while pregnant is not recommended.

decompression

cerebral decompression removal of a flap of the skull and incision of the dura mater for relief of intracranial pressure.

decompression sickness a condition resulting from a too-rapid decrease in atmospheric pressure, as when a deep-sea diver is brought too hastily to the surface. The popular term bends is derived from the bodily contortions its victims undergo when atmospheric pressure is abruptly changed from a high pressure to a relatively lower one. Called also caisson disease and divers' paralysis. A similar condition, altitude sickness, is suffered by aviators who ascend too rapidly to high altitudes. Decompression sickness may also be a complication in a type of oxygen therapy called hyperbaric oxygenation, in which the patient is placed in a high-pressure chamber to increase the oxygen content of the blood. Personnel and the patient within the chamber must be protected from decompression sickness when they emerge from the high-pressure chamber.

Cause. The phenomenon of decompression sickness is explained in terms of a law of physics: The greater the atmospheric pressure, the greater the amount of gas that can be dissolved in a liquid. The gas involved in this condition is the air we breathe, composed chiefly of nitrogen and oxygen. Under normal atmospheric pressure, nitrogen is present in the blood in dissolved form. If the atmospheric pressure is substantially increased, a proportionately greater amount of nitrogen will be dissolved in the blood. The same is true of oxygen, and this is the basis for hyperbaric oxygenation in the treatment of oxygen deficiency.

The increase in pressure causes no ill effects. Nor will there be any ill effects if the pressure is gradually brought back to normal. When the decrease in pressure is slow, the nitrogen escapes safely from the blood as it passes through the lungs to be exhaled. If the pressure drops abruptly back to normal, the nitrogen is suddenly released from its state of solution in the blood and forms bubbles. Although the body is now under normal air pressure, expanding bubbles of nitrogen are present in the circulation and force their way into the capillaries, blocking the normal passage of the blood. This blockage (or air embolus) starves cells dependent on a constant supply of oxygen and other blood nutrients. Some of these cells may be nerve cells located in the limbs or in the spinal cord. When they are deprived of blood, an attack of decompression sickness occurs.

The oxygen in the blood reacts similarly when abnormal pressure is abruptly relieved. But because oxygen is dissolved more easily than nitrogen, and because some of the oxygen combines chemically with hemoglobin, the oxygen released in decompression forms fewer bubbles, and is therefore less troublesome.

Symptoms and Treatment. Symptoms include joint pain, dizziness, staggering, visual disturbances, dyspnea, and itching of the skin. Partial paralysis occurs in severe cases; collapse and insensibility are also possible. Only rarely is decompression sickness itself fatal, although a diver while in this condition may suffer a fatal accident unless he or she is rescued. Treatment consists of placing the victim in a decompression chamber where the air pressure is at the original higher level of pressure. If the victim is a diver, this is the pressure at the depth where he or she was working. Pressure in the chamber is then reduced to normal at a safe rate.

de·com·pres·sion sick·ness

a symptom complex caused by the escape from solution in the body fluids of nitrogen bubbles absorbed originally at high atmospheric pressure, as a result of abrupt reduction in atmospheric pressure (either rapid ascent to high altitude or return from a compressed-air environment); it is characterized by headache; pain in the arms, legs, joints, and epigastrium; itching of the skin; vertigo; dyspnea; coughing; choking; vomiting; weakness (and sometimes) paralysis; and severe peripheral circulatory collapse. Bone infarcts can occur from bubbles in nutrient vessels leading to long-term consequences.See also: caisson sickness.

decompression sickness

n.

A disorder caused by the formation of nitrogen bubbles in the blood and tissues following a sudden drop in the surrounding pressure and characterized by joint pain, skin irritation, cramps, numbness, and, in severe cases, paralysis. It occurs especially in deep-sea divers when ascending rapidly from a dive. Decompression sickness is often called "the bends."

decompression sickness

a painful, sometimes fatal syndrome caused by the formation of nitrogen bubbles in the tissues of divers, caisson workers, and aviators who move too rapidly from environments of higher to those of lower atmospheric pressures. Nitrogen breathed in air under pressure dissolves in tissue fluids. When ambient pressure is reduced too rapidly, nitrogen goes out of solution faster than it can be circulated to the lungs for expiration. Gaseous nitrogen then accumulates in the joint spaces and peripheral circulation, impairing tissue oxygenation. Disorientation, severe pain, and syncope follow. Treatment entails rapid return of the patient to an environment of higher pressure (hyperbaric therapy) followed by gradual decompression. Death is more often caused by drowning during syncope than by decompression sickness itself. Also called bends, caisson disease, diver's palsy, diver's paralysis. Compare barotrauma.

de·com·pres·sion sick·ness

(dē-kŏm-presh'ŭn sik'nĕs)

A symptom complex caused by the escape from solution in the body fluids of nitrogen bubbles absorbed originally at high atmospheric pressure, as a result of abrupt reduction in atmospheric pressure (either rapid ascent to high altitude or return from a compressed-air environment); it is characterized by headache; pain in the arms, legs, joints, and epigastrium; itching of the skin; vertigo; dyspnea; coughing; choking; vomiting; weakness; possible paralysis; and severe peripheral circulatory collapse. Synonym(s): caisson disease.

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